Brake systems of this type, having different designs, are known in the prior art. The publication DE-36 01 914 A1, for instance, describes a dual-circuit hydraulic brake system with anti-lock and traction slip control, wherein the wheel brake of each wheel is connected to the brake system via a pair of inlet and outlet valves. For auxiliary-pressure supply during an anti-lock control operation or a traction slip control operation, a dual-circuit hydraulic pump is provided which is in operation only during a control action. In the traction slip control mode, the pump delivers, in two separate circuits, pressure fluid via pressure-limiting valves to the working chambers of the master cylinder which, in turn, are in communication with the pressure supply reservoir of the brake system via central valves. These pressure-limiting valves are connected in parallel with electrically controllable shut-off valves which, in their inactive position, connect the auxiliary-pressure supply system with the master cylinder.
These shut-off valves, with the parallel connected pressure-limiting valves, are inserted into the hydraulic system such that, after change-over of the shut-off valves, the non-driven wheels and, respectively, the associated inlet valves continue to be connected to the master cylinder, while the driven wheels are connected to the auxiliary pressure supply system. This is because only the brakes of the driven wheels are required for traction slip control. The direct pressure fluid conduit from the master cylinder to the wheel brakes of the non-driven wheels, which is kept open likewise in the traction slip control mode, makes possible a so-called braking intervention (brake actuation) during the TSC mode through these wheel brakes.
Further, it is known, even though not mentioned explicitly in the publication referred to hereinabove, to determine the pedal actuation or the pedal travel through travel switches or travel sensors, such as stop light switches, switches fitted to the master cylinder piston etc., and to generate corresponding electric signals. These travel-responsive signals are required for controlling and monitoring controlled brake systems. The difficulty encountered in brake systems of the type described hereinabove is that the "braking intervention" during the TSC mode, in consequence of the switched-on auxiliary-pressure supply system and the constant delivery of pressure fluid into the master cylinder, causes pressure to develop very quickly in the working chambers of the master cylinder which permits no pedal movement at all or only insufficient pedal movement. A safe response of the travel-responsive switches or sensors, therefore, is not assured. A mutual monitoring of several switches in respect of their operability is not possible. In order to recognize the brake pedal actuation without any doubt, it has proven necessary to sense the braking pressure in the master cylinder chambers in addition to the pedal travel. The complexity and cost of such an arrangement is a major disadvantage.